1. Field of the Invention
The present invention relates to a technology for judging that a shut-off valve used in a vehicle fuel gas supply system is in a failure state.
2. Description of the Related Art
In recent years, natural gas (NG) has been used as one of alternative fuels to gasoline and gas oil, and in a case where natural gas is used as fuel for vehicles, in particular, passenger vehicles, a compressed-gas cylinder (bomb) is mounted on a passenger vehicle. A natural gas compressed to in the order of 200 kg/cm2 is then filled in the compressed-gas cylinder, and the natural gas so filled is then reduced in pressure by means of a pressure reducing valve to become a low pressure gas for supply to a combustion chamber. A natural gas as compressed to a high pressure like this is called a compressed natural gas (CNG).
Research on vehicles using compressed natural gas as a fuel has been progressing, and Japanese Patent Unexamined Publication No. Hei.7-189731 entitled xe2x80x9cResidual Fuel Volume Display Device for Gas-Fueled Vehiclesxe2x80x9d proposes a technology for displaying the volume of residual fuel more accurately by correcting pressure and temperature fluctuations attributed to a compressible fluid. This publication discloses a CNG tank, a high pressure piping for supplying a compressed natural gas taken from the CNG tank, a first solenoid shut-off valve provided upstream of the high pressure piping, and a second solenoid shut-off valve provided downstream of the high pressure piping. The first and second solenoid shut-off valves are able to be closed by means of an ECU.
In general, used as the solenoid shut-off valves is a shut-off valve in which the valve is opened when a plunger (rod) is axially moved by virtue of the electromagnetic force of the solenoid, while the valve is closed when the plunger is returned by virtue of the compression force of a spring used. In order to allow the plunger to reciprocate in the axial directions, there must be provided a gap. However, if this gap is too large, the plunger may be moved in radial directions, causing xe2x80x9cchattering.xe2x80x9d To cope with this, the gap between the plunger and a component for supporting the plunger thereon needs to be as small as possible.
Natural gas consists mainly of methane gas, and since it is collected from the underground, the natural gas tends to contain water. Even if much labor is spent in producing a dried compressed natural gas, there exists a risk of water contained in air entering into a fuel tank mounted on a vehicle when the dried compressed natural gas is transferred into the fuel tank. In addition, it is well considered that water cannot be removed in a complete fashion even in producing a fuel line system.
There is a risk of the water so present generating rust at metallic portions of the solenoid valves. To cope with this, plungers are made of stainless steel or plated, but those countermeasures are not perfect.
There are considered the following two cases of failure in operation of the first solenoid shut-off valve.
The first case is where rust disturbs to open the shut-off valve cannot be opened because of rust. With the shut-off valve being left closed, a sufficient amount of fuel cannot be supplied to the engine, and therefore, although the engine can be started with fuel remaining within the fuel line, the engine stops soon thereafter.
The second case is where the shut-off valve from cannot be closed because of rust. With the shut-off valve being left opened, even if the engine is stopped, the compressed natural gas is kept supplied from the compressed gas cylinder to the high pressure piping, and therefore, there is a high risk of gas leakage while the engine is being stopped. In addition, in view of safety servicing of a vehicle, such a gas leakage is undesirable.
Then, an object of the present invention is to provide a technology for efficiently judging of a failure state of a solenoid shut-off valve used in a vehicle fuel gas supply system having a high probability of a risk of water entering into the fuel supply system.
With a view to attaining the aforesaid object, according to the present invention, there is provided a vehicle fuel gas supply system in which a shut-off valve and a pressure sensor are disposed in that order along a fuel line for supplying a fuel gas to a gas engine, comprising a control section for closing the shut-off valve based on a diagnostic signal, calculating a pressure drop rate based on pressure information taken in from the pressure sensor after the closure of the shut-off valve and an elapsed time, and judging that the shut-off value is in a failure state when the pressure drop rate so calculated is smaller than a predetermined pressure drop rate threshold value.
With this fuel gas supply system constructed as described above, it is possible to judge of a failure state of the shut-off valve while the vehicle is being stopped or running.
The diagnosis of failure consists of the following two types; the shut-off valve is kept opened and cannot be closed, and the shut-off valve is locked to the opened side and a sufficient degree of closure cannot be attained.
According to the present invention, it is possible not only to diagnose that the shut-off valve properly operates at suitable times but also to prevent a trouble that would be entailed by the failure of the shut-off valve.